19 % 的MgCl 2 溶液中，Cl－与H 2 O个数比为 （ ）
A．4∶90
B．4∶45
C．2∶45
D．19∶81
答案：B
Entering the room, I found Mary _______ at the desk and _______ a letter.
A．seated; wrote
B．sitting; written
C．seating; writing
D．seated; writing
信息资源组织的概念？
